1. Field of the Invention
The present invention relates to, for example, a digital broadcasting system such as terrestrial digital television broadcasting.
2. Description of the Related Art
The terrestrial digital television broadcasting system which has been currently operated covers a desired service area through a transport stream transmitter transmitter link (TS-TTL) transmission network, intermediate frequency transmitter link (IF-TTL) network, and a broadcast wave receiving/retransmitting network. Meanwhile, in an initial development phase of the terrestrial digital television broadcasting system, application of a single frequency network (SFN) has been intended to effectively utilize a frequency. However, in actual, it is hard to fully shorten the distance between a transmitting station and a relay station, and a signal delay time difference between two signals in a receiving area shared with both stations exceeds a guard interval. Thereby, it is impossible to satisfy one condition of the SFN that is “signal delay time difference between two signals in receiving area should be within guard interval”. In the relay station, it is hard to secure isolation between a reception antenna and a transmission antenna in many cases, and this case results in an obstacle of achieving the SFN.
As mentioned above, the construction of a broadcasting network through the SFN, which is planned in an initial phase, has been in a technically difficult situation, and at present, a multi-frequency network (MFN) covers main service areas. Therefore, effective utilization of frequencies is spoiled, and effect to switch analog broadcasting to digital broadcasting is reduced by half. Since it is impossible to distinguish the signal from which broadcasting station the receiving signal is transmitted when the SFN is established, such a problem that the broadcast wave in the service area cannot be optimized has been posed.
As to a technique for maintaining a reception level in a service area constant, Jpn. Pat. Appln. KOKAI Publication 2000-324361 discloses a technique to measure, by a field intensity measuring instrument, a reception level of a transmission signal transmitted from a transmission antenna, to adjust the reception level of the modulation signal by a step attenuator on the basis of the measurement result, and to control the output of the transmission signal to be transmitted constant in a digital television device to apply level adjustment to an OFDM-modulated television signal by means of the step attenuator, and to power-amplify it by a power amplifier to transmit it from the transmission antenna.
As described above, in the conventional digital broadcasting system, since it is hard to construct a broadcasting network through the SFN in a relay transmission, and it is impossible to distinguish that from which broadcasting station the reception signal is transmitted when the SFN is established, there is a problem such that the broadcast wave in the service area cannot be optimized.